To marine structures, such as ships, bridges, tanks floating on the sea and the like, and marine fixed facilities, such as fish culturing nets, trap nets and the like, a large number of marine organisms, such as barnacles, tube worms, oysters, ascidians, Bryozoa, sea lettuce and green laver, deposit and cause great damage, such as corrosion of the structures, reducing of sailing speed of ships and death of a large number of fishes due to poor flowing of sea water resulting from clogging of meshes of the net. Therefore antifouling paints are generally used in order to prevent the attachment of troublesome marine organisms. However, the conventional antifouling paints have a short antifouling period said period, being only 12-16 months so that the antifouling paint must be coated again. Thus antifouling paints having a long period of activity have been demanded.
The antifouling paints are classified into two groups in view of the mechanism of the antifouling function.
One group is referred to as the "soluble matrix type". When the resin which is the vehicle or matrix is dissolved in sea water, an antifouling agent, such as cuprous oxide dispersed in the matrix is also dissolved off to maintain the concentration of the antifouling agent in sea water near the coating film at a concentration higher than the lethal dose for marine organisms, whereby the antifouling object can be attained. The other group is referred to as the "insoluble matrix type"; the matrix is formed of resins insoluble in sea water, such as vinyl chloride, chlorinated rubber, styrene-butadiene polymer and a component soluble in sea water, such as rosin. When this insoluble matrix type antifouling coating film is dipped in sea water, the rosin is dissolved off in sea water and simultaneously the antifouling agent dispersed in the matrix is dissolved off and the concentration of the antifouling agent in sea water near the coating film is maintained at a concentration higher than the lethal dose for marine organisms and the antifouling object can be attained.
In the former soluble matrix type, rosins or fatty acids form the matrix, but these substances have a high dissolution rate in sea water and the consumption of the coating film is severe, so that it is impossible to effect the antifouling for a long period of time; further, the matrix is of low molecular weight, so that the coating film is low in strength and is too soft and brittle and is difficult to use in a thick coating. Accordingly, the insoluble matrix type, wherein the resin insoluble in sea water is used as a coating film reinforcing agent, occupies the major part of the commercially available antifouling paints at present.
In the insoluble matrix type, the original dissolution rate of the antifouling agent into sea water is high, but when the section of the coating film dipped in sea water for a few months is observed by a microscope and analyzed, at the upper layer portion of the coating film, only the insoluble resin remains and the lower layer portion contains the insoluble resin, rosin and the antifouling agent, and maintains the same state as the state prior to the dipping. In such a state, the insoluble resin residue in the matrix of the upper layer portion prevents the dissolution of antifouling agent and rosin and the rate of the antifouling agent dissolved off gradually lowers. When 12-16 months pass after the dipping, even though a sufficient amount of antifouling agent remains in the lower layer portion, the rate at which the antifouling agent dissolves off lowers and the dissolution becomes insufficient and the concentration becomes less than the lethal dose for marine organisms and the organisms start depositing.
As mentioned above, the dissolution rate is too high in the soluble matrix type, and the thick coating is difficult; in the insoluble matrix type, a long period of antifouling is impossible due to the insoluble resin residue, so that both types are incapable of preventing fouling for a long period of time.
From the above described points, a soluble matrix type antifouling paint which can provide the antifouling property for a long period of time, the coating film of which has a high strength, and which is moderately soluble in sea water and can be coated in a thin layer, is most desirable.
As inventions having such an intention, mention is made of Japanese Patent Application Publication No. 21,426/65, No. 9,579/69 and No. 12,049/76, which disclose antifouling paints.
In these inventions, polymers obtained either by homomopolymerizing an organotin compound monomer having the general formula ##STR1## or copolymerizing said monomer with other unsaturated compounds, form the matrix. When the antifouling paint contacts sea water, a hydrolytic decomposition reaction occurs to separate the matrix into the organotin compound of the antifouling agent and a polymer containing a carboxyl group. This polymer is dissolved in sea water and therefore this antifouling paint is classified in the soluble matrix group.
However, synthesis of the organotin compounds having an unsaturated group is difficult, their storage stability is poor and they tend to increase in viscosity, and they are dissolved in sea water owing to hydrolysis, so that the polymers are sensitive to the pH of sea water and the dissolution rate varies depending upon the sea area.
Accordingly, as the process which provides the solubility to the polymer of the matrix without utilizing a hydrolytic decomposition mechanism, consideration has been given to introduction of hydrophilic groups, such as free carboxyl groups or hydroxyl groups into the polymer. However, these hydrophilic groups are apt to react with metallic antifouling agents, such as cuprous oxide, tributyltin compounds, triphenyltin compounds and the like, at room temperature and a cross-linking reaction occurs in a vessel to cause gelation and the desired use becomes impossible.